Comparison between Computed Tomography and Ultrasonography in Detecting Foreign Bodies Regarding Their Composition and Depth: An In Vitro Study.

STATEMENT OF THE PROBLEM
Impaction of foreign bodies in the soft tissues is a sequela of traumatic and penetrating injuries. Such foreign bodies should be removed due to the complications they cause. Patient's history, clinical evaluation and imaging examinations aid in the proper detection and localization of the foreign bodies.


PURPOSE
The aim of the present study was to compare the sensitivity of computed tomography (CT) and ultrasonography for detecting foreign bodies in in-vitro models simulating facial soft tissues.


MATERIALS AND METHOD
Fifty foreign particles with five different compositions including wood, glass, metal, plastic, and stone were embedded in five calf tongues at 1, 2, 3, 4 and 5 cm depths. CT and ultrasonography were compared regarding their capability of detecting and localizing the foreign bodies.


RESULTS
Wood and plastic foreign bodies were demonstrated more clearly on ultrasonography images. High density materials such as metal, stone, and glass were detected with almost the same accuracy on CT and ultrasonography examinations. Visibility of the foreign bodies deteriorated on ultrasonography images as their depth increased; however, CT appearances of the foreign particles were not influenced by their depths.


CONCLUSION
Ultrasonography is an appropriate technique for detection of foreign bodies especially the ones with low density. Therefore, it seems logical to perform ultrasonography in combination with CT in cases with the suspicion of foreign body impaction.


Introduction
Foreign bodies are objects originating outside the body.
Most often they are retained in body tissues due to car accidents, explosions and gunshot injuries and further complicate the patients' situation. [1][2] Oral and maxillofacial surgeons frequently come upon foreign bodies.
Factors such as size, difficult access, and close anatomic relationship of the foreign body to vital structures can present diagnostic challenges to the surgeons. [3][4] Foreign bodies are either inert or irritating. The irritating ones cause inflammation, infection, abscess formation, pain and scarring. [5] Furthermore, they can obstruct pathways either by their size or by the scarring they cause. Besides, some foreign bodies are toxic. [6] Location and composition of the foreign bodies can vary considerably based on their route of entrance into the body tissues. [1] Regarding their composition, the most frequent foreign bodies are wood, glass and metal. [2,7]  In addition, there is increasing number of reports related to stone foreign bodies in maxillofacial surgery. [8] Removal of foreign bodies can be delayed in approximately one third of all cases due to initial radiographic missing or misdiagnosing. [9] Therefore, selecting an appropriate imaging technique is crucial for proper recognition of foreign bodies. Several imaging modalities including conventional plain radiography, computed tomography (CT) scans, ultrasonography and MRI have been evaluated in vivo and in vitro for locating foreign bodies. [10][11][12][13][14] MRI seems to be the least suitable method as particles with metallic contents at times could have hazardous movements due to the strong magnetic field. [10,15] Furthermore, foreign bodies of almost all compositions are seen as low signal areas on MR images; thus, appearing indistinguishable from structures such as calcifications and scar tissues.
[2] CT and in particular ultrasonography have been proved to be appropriate for foreign body detection in soft tissues. [7,16] Ultrasonography has been shown to be an accurate method for detection and localization of radiolucent foreign bodies. [5,16] It has been postulated that superficial foreign bodies with low density are detected more effectively by ultrasonography than CT and conventional plain radiography. [1] Considering the high patient exposure dose in CT and the concerning that CT is hypothesized to be not as effective as ultrasonography in detecting low-density foreign bodies, the present study was conducted to compare CT and ultrasonography in detecting wood, plastic, glass, stone and metal foreign bodies. Moreover, the effect of impaction depth of the foreign bodies on their visibility was also evaluated.

Materials and Method
Fifty particles with five different compositions including wood, glass, metal (stainless steel), plastic (acrylic sheet), and stone were used as foreign bodies for this in vitro study (Figure 1).
The rationale for selection of the mentioned materials was that they are the most frequent foreign bodies retained in human tissues. All particles had volumes in the range of 40-45 mm 3 . Initially, radiodensities of the particles were determined in Hounsfield Units (HU) by means of a CT scanner (GE VCT; General Electric, United States). Table 1 shows the HUs of the substances. Five fresh calf tongues were used as representatives of maxillofacial soft tissues in the present study.
All examinations were performed one day after the calves' death. We intended to evaluate the visibility of particles at 1, 2, 3, 4 and 5 cm depths. In each of the tongues, ten particles were placed in two separate rows.
Each row contained five objects with different compositions. The two rows in each tongue were created at different depths. The incisions were made by using a scalpel and sutured in order to fix the particles and cover their surfaces with soft tissue.  (Table 2).

Wood
Wooden foreign bodies were well recognized by ultrasonography up to the depth of 4 cm; however, CT was unable to detect wood particles ( Figure 2).

Metal
Metal particles embedded at less than 4-cm depths were detected with the same accuracy in CT and ultrasonography (Figure 4a and b).

Comparative statistical analysis of CT and ultrasonography
Regardless of the depth of the foreign bodies, Wilcoxon signed-ranks test revealed that the visibility of wood and plastic in ultrasonography was significantly superior to CT. On the other hand, CT was more efficient in detecting glass particles compared to ultrasonography. No significant differences existed in the visibility scores of metal and stone foreign bodies between CT and ultrasonography. Table 5 demonstrates the comparison between CT and ultrasonography in detecting foreign bodies regardless of their depth.  Table 6 presents the relationships between foreign bodies' impaction depths and their visibility scores in ultrasonography and CT.

Discussion
Impaction of foreign bodies in the soft tissues is a sequela of traumatic and penetrating injuries. [17][18] Such foreign bodies should be removed as they can interfere with the healing process of the tissues; [7] therefore, proper detection and localization of them are imperative. The efficacy of foreign body detection depends on its composition, size and location. [17] Definitely, the employed imaging method is also of great importance.
Investigations regarding foreign body detection usually use in vitro models. [7,14,[19][20] because in studies performed in vivo, the examiners often have knowledge of other imaging results and there is inadequate control over the size, composition and location of the foreign bodies. [21] There are also some problems with in vitro models including lack of the ability to in duce inflammatory reactions and other body responses [2] CT allows for the precise localization of the foreign bodies as a prerequisite for surgical removal; [12,[26][27] however, some studies reported that low-density foreign bodies are not detected effectively by CT. [1,7] Ultrasonography has emerged as an appropriate imaging modality due to its widespread availability, relatively low cost, and the reported 95% sensitivity for foreign body detection. [20,[28][29]As they appear to be the most applicable imaging modalities, CT and ultrasonography were used for foreign body detection in the present investigation. anchored "no image" to (0) and "excellent image" to (4). Our scoring scale allocated "no image" to (0), "bad image" to (1), "fair image" to (2) and "good image" to (3). In our scale, resolution of the foreign body image and its demarcation with the surrounding tissues were considered as the assessment criteria. Moreover, the (4) score was omitted compared to the previous studies as the distinction between the "excellent image" and "good image" is fairly subjective. This is not a problem with the other scores, as clear distinction could be made among "no image", "bad image", "fair image" and "good image" regarding the resolution and demarcation of the foreign body image. However, "good image" and "excellent image" are so indiscriminate that such a distinction could not be performed confidently.